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Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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419 USER COMMENTS

Chez, I suggest using an epoxy because of its strength and adhesion. Sakrete does not manufacture a product that will work for this type of repair.
- Chris Technical Services
Tuesday, December 16, 2014 at 10:55 AM

I am trying to repair old cement urns. The stems have come off and I want to stick them back on. What should I use and how will I do this?
- Chez
Monday, December 15, 2014 at 8:05 PM

Finn, we recommend to wait the full 28day cure time if the 24 hour green period is exceeded. Sakrete Bonder and Fortifier can be used as a primer and admixture. If used as a primer you should apply a thin coat to the clean concrete surface using a paint roller or low pressure pump sprayer. Coverage will vary due to porosity and texture of the substrate.
- Chris Technical Services
Thursday, December 11, 2014 at 11:27 AM

Tim, Sakrete Self Leveling Underlayment is a polymer modified Portland cement and gypsum based underlayment for interior applications. You will need to properly prepare the surface by removing all dirt, oil, grease, paint, stain, sealers, etc. The surface should be scarified and primed using Sakrete Bonder and Fortifier. To achieve the required 2” depth multiple lifts will be necessary. Please refer to our published technical datasheet for preparation and placement details.
- Chris Technical Services
Thursday, December 11, 2014 at 10:31 AM

Do I really have to wait 28 days for sand mix to cure before I can start tiling (thinset or similar)? You calculator does not include Sakrete Bonder Fortifier. I assume the most economical is just adding it to scratchcoat of sand mix. How much Sakrete Bonder Fortifier do I need for 80 sq ft?
- Finn L
Thursday, December 11, 2014 at 8:10 AM

Hi – I have a house on slab and recently put an addition on the entire back of the house. The newly poured concrete slab for the addition is level, however, the slab in the original structure is not level. The original slab (approx 1,000 sqft) slopes / sinks into the center of the slab. At the lowest section (the middle of the original slab), it’s 2” lower than the perimeter. My goal is to finish the entire 1st floor with wood flooring (engineered wood floors) but I need to glue them to a level slab. It’s my understanding that I can’t use a self leveling product because the thickness of 2” exceeds the products thickness specifications, and it’s extremely expensive. I don’t want to break the remodel budget, solely on leveling the existing slab. That said, my questions are ….. 1) How & what products do you recommend I use to level the existing slab? … 2) What product(s) should I be using to bond whatever I’m using to level the floor to the existing slab? … 3) Does it help to chop up the existing slab in some sections so the leveling product bonds to the existing slab? … 4) Should I be using a wire mesh to strengthen the material I’m using to level the floor? … 5) What is the cure time for whatever process you are recommending? Any professional insight / recommendations would be greatly appreciated … Thanks!
- Tim
Wednesday, December 10, 2014 at 3:37 AM

Merv, to ensure that you get a quality bond with the existing concrete you will first need to prep the surface by cleaning and removing any bond breakers such as paint, stain, grease, oil, etc. Sakrete Sand Mix and Sakrete Bonder and Fortifier together will provided a tight bond with your existing floor. Sakrete Sand Mix can be used for repairs requiring up to 2” down to ½”. Sakrete Bonder and Fortifier can be used as a primer and also an admixture. Remember that a garage should be sloped ¼” per foot away from the foundation for proper drainage. Also honor any expansion joints. Finally to finish, use Sakrete Flo-Coat from ½” down to a feathered edge to provide a smooth transition.
- Chris Technical Services
Tuesday, November 18, 2014 at 9:54 AM

I would like to level a concrete shop floor. One corner has dropped 1.5 inches,I would like to correct it,leveling and feathering it towards the overhead garage door. The motorcycle storage shop is 9' X 12',and has been constructed on our concrete drive.
- Merv Knechtel
Sunday, November 16, 2014 at 3:35 AM

Roycellen, are you looking to add a lip to your existing patio? Stem walls are usually made with cinder blocks reinforced with rebar and concrete.
- Chris Technical Services
Tuesday, November 11, 2014 at 10:46 AM

I need to pour a 6 inch stemwall on existing patio concrete how do I keep it from leaking thru when it is finished
- Roycellen
Tuesday, November 4, 2014 at 8:04 AM

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